The present invention relates to systems and methods for starting an engine and specifically to a system and method for reducing undesired effects resulting from an initial-engagement of a starter to an engine.
Presently, various methods exist for starting engines. Engines such as diesel and gasoline combustion engines, and gas turbine engines must be externally rotated at a sufficient speed before they are self-sustaining. A starter such as an electric motor starter or an air starter is typically coupled to the engine through a clutch and is used to rotate the engine. The clutch engages the starter to the engine when the engine is started and disengages the starter from the engine when the engine is running. For example, most automobile engines incorporate an electric motor starter that is coupled to the engine through an overrunning clutch. When electric power is supplied to the electric motor starter from a battery, the electric motor starter begins to rotate which causes a gear on the motor shaft to engage a drive gear on the engine. The electric motor starter rotates the engine until the drive gear on the engine rotates faster than the gear on the electric motor starter, at which point the gear on the electric motor starter retracts away from the drive gear to disengage the electric starter motor from the engine.
Conventional starting systems, however, have several undesired effects resulting from the initial engagement between the starter and the engine. When the starter initially engages the engine, the inertia of the engine resists free rotation of the starter causing the starter to be stalled for a brief period of time. Mechanical components such as the clutch, gears and bearings can experience stress as a result of the high-impact forces produced by the initial engagement of the high-speed motor and the non-rotating engine. Further, in typical starting systems employing electric motor starters, the initial engagement causes high electrical currents to be drawn from the power supply. In addition to placing performance demands on the power supply, these high currents contribute to excessive brush heating in the electric motor starters.
When current is passed through the armature of a DC starter motor, the resulting magnetic field generates a torque, causing the armature to rotate. The revolution of the armature induces a counter electromotive force (emf) voltage in the armature windings that is opposite in polarity to the voltage applied to the armature by the power supply. The xe2x80x9cbackxe2x80x9d emf voltage is directly proportional to the speed of the motor. The emf voltage is almost equal to the applied voltage and the current is relatively low at high speeds. At low speeds, the counter emf is low. Typically, the resistance of the armature winding and brush circuit is also low. This combination of conditions results in higher currents. The commutator bars in the motor expand as a result of the heat generated by the high current, causing the heated bars to expand beyond the diameter of the remaining commutator. As the commutator begins to rotate against the carbon brushes, the carbon brushes are scraped by the extended copper bars resulting in undesired brush wear and a decreased life of the electric motor starter.
These undesired effects are especially significant in auxiliary power units (APUs) used in aircraft. APUs provide electrical, hydraulic and pneumatic power to the aircraft when the aircraft is on the ground. In addition, the APUs may be required to provide any combination of these powers in emergency situations while the aircraft is in flight. Accordingly, the gas turbine engines used in APUs must be designed and maintained to start at high altitudes where air is oxygen poor and temperatures are extremely low. These extreme requirements, coupled with the potential loss of life, often require the APUs to be in optimum condition at all times. In addition, removal of the APUs from the aircraft is expensive and time consuming.
Electric motor starters used in starting systems for gas turbine engines are particularly susceptible to high inrush currents and excessive brush wear. The rotational speed of a gas turbine engine must be increased to approximately 60% of the engine""s normal operating speed before the engine becomes totally self-sustaining. One known starting method involves the use of an electric DC starter motor directly coupled to the gas turbine engine through a gear and clutch assembly. Battery power is applied to the electric starter motor to produce a torque at the output of a drive shaft of the starter motor. The torque is coupled to the engine through a drive gear on the engine until the engine has started. Conventional gas turbine engine starter designs utilize high speed direct current (DC) series wound electrical motors. Due to the inertia of the APU and gearbox drag, electric motors cannot provide the necessary torque to rotate the gas turbine engine when the starter initially engages the gas turbine engine. Accordingly, extremely high currents flow through the armature windings and commutator bars when the electric motor starter is briefly stalled by the rotational inertia of the gas turbine engine. In some conventional 28V DC APU systems, the inrush current in the armature windings may be as high as 900 amperes resulting in significant brush wear. The component stress and brush wear resulting from the initial engagement between the starter and the gas turbine engine require the APU starters to be frequently inspected and replaced.
Therefore, there exists a need for an engine starting system that is less susceptible to the impulse forces and thermal effects which are encountered during the initial engagement between the starter and the engine.
The present invention relates to an engine starting system and method for reducing undesired effects resulting from the initial engagement of the starter to the engine.
In an exemplary embodiment, an electric motor starter is coupled to a gas turbine engine through a spiral spring and an overrunning clutch. A first end of the spiral spring is connected to a drive shaft of the electric motor starter and a second end of the spiral spring is coupled through a spring hub by the overrunning clutch. When a supply voltage is applied to the electric motor starter from a battery, the motor begins to rotate. Initially, the overrunning clutch engages the engine and the second end of the spiral spring is briefly motionless due to the inertia of the engine. Rotational energy is stored in the spiral spring as the motor rotates. The motor continues to rotate as rotational energy is transferred form the spiral spring and motor to the engine. The rotational speed of the engine increases as more rotational energy is transferred to the engine. When the engine is self-sustaining and rotates faster than the starter, the overrunning clutch disengages from the engine.
Therefore, the electric motor starter is allowed to rotate during the initial stages of the engine starting sequence while forces due to the high impact engagement are minimized. The rotational energy from the electric motor starter is stored in the spiral spring and is gradually transferred to the engine. Since the motor is coupled to the engine through the spiral spring, initial duration of current through the brushes, armature windings and the commutator bars of the motor is reduced. Further, the wear on the carbon brushes is minimized since the temperature increase of the commutator bars is reduced.